Polythermal Metamorphism of Limestones at Kilchoan, Ardnamurchan

S. O. Agrell

Department of Mineralogy and Petrology, Cambridge

Summary: Jurassic limestones occurring in a screen between two ring dykes are shown to have undergone two periods of thermal metamorphism; the first charac-terized by a high ratio of CO2:H2O and the second by a high ratio of H2O:CO2 in the vapour phase. The first metamorphism produced minerals characteristic of Bowen's decarbonation series. Some of these minerals became unstable in the second metamorphism and (OH, F) bearing minerls were produced. A stage of fracture-controlled replacement veins with mineral assemblages which indicate a varying CO2:H2O ratio in the vapour and a general falling temperature sequence terminate the metamorphic history. Three new minerals, rustumite (Ca3Si2O7.Caterminate (OH)2), dellaite (Ca12Si6O22(OH)4), and kilchoanite (Ca3Si2O7), of which the last has previously been described, are characteristic of the second episode of meta-morphism.